In a liquid crystal display (LCD), a backlight control circuit is employed to control light emitting diodes (LEDs) to illuminate from the back side of the liquid crystal display, which enables a user to observe an image from the front side of the liquid crystal screen.
According to state of the art, there are two types of arrangements for the backlight LED structure, one of which employs single-color white LEDs, and the other of which employs red, green and blue (RGB) LEDs. The latter is referred to in this specification as “multi-color backlight”, and the control circuit thereof is referred to as “multi-color backlight control circuit”. Single-color white LED backlight requires a less sophisticated control circuit, but the “white” light generated is not true white light; it is actually a synthetic light having less light quality by exciting fluorescence powders by blue LEDs. On the other hand, white light obtained by mixing the lights from R, G and B LEDs has better light quality. However, regardless whether the white backlight is obtained from white LEDs or from R, G and B LEDs, the light has to pass through color filters in the LCD, and whatever portion of the light not consistent with the color of the filters is filtered out. In other words, there is energy loss and the photo energy is not utilized to the best.
A so-called “color sequential technique” is proposed to deal with the above issue, in which the R, G and B LEDs sequentially emit light in correspondence with the pixels of the same color in the LCD, so no color filters are used. The technique saves power, but requires a more sophisticated control circuit. Thus, a multi-color backlight control circuit adapted to this color sequential technique becomes very important and is very much desired.
More specifically, the operational voltages of the R, G and B LEDs are different. In general, a white LED has an operational voltage of about 3.2V-3.8V; a red LED has an operational voltage of about 1.9V-2.6V; a green LED has an operational voltage of about 2.9V-3.7V; a blue LED has an operational voltage of about 3.0V-3.8V. In the application of LCD backlight, it requires to connect a considerable number of LEDs in series, and therefore the supplied voltages for strings of LEDs of different colors are greatly different, probably more than 15 volts in a practical application. Hence as shown in FIG. 1, the prior art arrangement provides three backlight control circuits 10R, 10G and 10B to supply three different voltages Vout(R), Vout(G) and Vout(B), for controlling the brightness and power efficiency of R, G and B LEDs respectively. The three backlight control circuit may be integrated in one circuit chip, but it still requires to duplicate three voltage supply circuits and corresponding feedback control circuits.
The prior art structure is apparently not optimum. Thus, it is desired to provide a more efficient multi-color backlight control circuit with simpler hardware structure and lower cost.